In the manufacture of a flat panel display such as an LCD (liquid crystal display) and a semiconductor device, plasma processing systems are widely used to perform processes such as etching, ashing, and CVD (Chemical Vapour Deposition). Among the plasma processing systems, a microwave plasma processing system is available which supplies microwaves into a processing vessel to ionize, excite, or dissociate a gas in the processing vessel, thus generating a plasma.
FIG. 11 is a longitudinal sectional view showing the overall arrangement of a conventional plasma processing system. This plasma processing system has a bottomed cylindrical processing vessel 1 which is square when seen from the top. The processing vessel 1 is made of a metal such as Al. A stage 2 is disposed at the central portion of the bottom surface of the processing vessel 1. An LCD substrate 3 or the like is arranged as a target object on the upper surface of the stage 2. The stage 2 is connected to a high-frequency power supply 5 through a matching box 4.
Exhaust ports 6 for vacuum evacuation are formed in the peripheral portion of the bottom surface of the processing vessel 1. A gas introduction port 7 through which a gas is introduced is formed in the side wall of the processing vessel 1. When the plasma processing system is to be used as an etching system, a plasma gas such as Ar and a reaction gas such as CF4 are introduced.
The upper opening of the processing vessel 1 is closed with a dielectric plate 8 made of silica glass or the like, so the reduced pressure in the processing vessel 1 is maintained while introducing microwaves through the upper opening. An O-ring is interposed between the upper surface of the side wall of the processing vessel 1 and the dielectric plate 8 to ensure hermeticity in the processing vessel 1.
A microwave supply device 110 which supplies microwaves into the processing vessel 1 is arranged above the dielectric plate 8.
FIG. 12 is a cross-sectional view showing the arrangement of the conventional microwave supply device 110. The microwave supply device 110 comprises a microwave oscillator 120, microwave distributor 130, and antenna assembly 150.
The antenna assembly 150 has a plurality of radiation waveguides 151A, 151B, 151C, 151D, 151E, and 151F. Each of the radiation waveguides 151A to 151F is formed of a square waveguide. A square waveguide is a waveguide whose section perpendicular to its axis is rectangular, and can transmit microwaves with a transmission mode TE10. With the transmission mode TE10, a magnetic field is formed parallel to the tube wall, i.e., the wide wall, including the long side of the section, and an electric field is formed parallel to the tube wall, i.e., the narrow wall, including the short side of the section. A plurality of radiation slots 152 are formed in the wide wall of each of the radiation waveguides 151A to 151F. Microwaves input to each of the radiation waveguides 151A to 151F are radiated into the processing vessel 1 through the radiation slots 152 and used to generate a plasma.
A wave delaying member 153 made of a dielectric (with a relative dielectric constant ∈r (>1)) is arranged in each of the radiation waveguides 151A to 151F. This renders a tube wavelength λg to 1/(∈r)1/2, so that the distance among the radiation slots 152 which is set on the basis of the tube wavelength λg is shortened to uniform the distribution of the plasma density.
The microwave distributor 130 has a square microwave waveguide 131 and square feeding waveguide 141 and distributes the microwaves input from the microwave oscillator 120 to the radiation waveguides 151A to 151F of the antenna assembly 150. The input end of the microwave waveguide 131 is connected to the microwave oscillator 120, and its output end is connected to the central portion of one narrow wall 141A of the feeding waveguide 141. A communication hole 132 through which the two microwave waveguides 131 and 141 communicate with each other is formed at the central portion of the narrow wall 141A (for example, see Masamitsu Nakajima, “Microwave Engineering”, Morikita Shuppan, p. 132). A plurality of openings 143 through which the feeding waveguide 141 communicates with the radiation waveguides 151A to 151F are formed in the other narrow wall 141B of the feeding waveguide 141.
If some openings 143 oppose, even partly, the communication hole 132 which allows communication with the microwave waveguide 131, the microwaves to be output to the radiation waveguides through the openings 143 increase, and distribution of the microwaves among the radiation waveguides 151A to 151F becomes nonuniform. Irises 147A and 147B are formed at the connecting portion of the microwave waveguide 131 and feeding waveguide 141 to narrow the width of the communication hole 132 to be smaller than the tube width of the microwave waveguide 131 so as not to make the communication hole 132 which allows communication with the microwave waveguide 131 oppose the openings 143 which allow communication with the radiation waveguides 151A to 151F.
The interior of the microwave waveguide 131 is hollow, and the wave delaying member 153 is arranged in the feeding waveguide 141.
As described above, in the conventional plasma processing system, since the irises 147A and 147B are formed at the connecting portion of the microwave waveguide 131 and feeding waveguide 141, the tube width is narrow at the connecting portion, and the band of the frequency that can pass through the connecting portion becomes narrow. In particular, as in a case wherein the hollow microwave waveguide 131 is to be connected to the feeding waveguide 141 in which the wave delaying member 153 is arranged, the larger the difference in relative dielectric constant in the tube, the more typical the narrowing tendency of the frequency band becomes. Therefore, when the oscillation frequency of the microwave oscillator 120 changes only slightly, power that cannot pass through the connecting portion of the two waveguides 131 and 141 but is reflected increases. Thus, reflection loss in the microwave distributor 130 comprising the waveguides 131 and 141 increases.